Heretofore, as illustrated in FIG. 1, a compressor 1 and an expander 2 have been provided in a compression side, i.e., a transmitting side, and in an expansion side, i.e., a receiving side, respectively. The compressor 1 comprises variable gain amplifiers (VCA) 3 and 4, a peak detector OR circuit 5, and a time constant circuit 6. The peak detector OR circuit 5 is connected to the output end of each of the VCAs 3 and 4, and is used to detect which one of the output levels of VCAs 3 and 4 is greater, and then supplies the detected greater level through the time constant circuit 6 to the gain-controlling end of each of the VCAs 3 and 4. A transmitter 8 is connected through a matrix circuit 7 to the output ends of the compressor 1, i.e., the output ends of VCAs 3 and 4.
Similarly to the compressor 1, an expander 2 comprises VCAs 9 and 10, a peak detector OR circuit 11, and a time constant circuit 12. The peak detector OR circuit 11 is connected to the input end of each of th VCAs 9 and 10 so that the greater level detected by the peak detector OR circuit 11 is supplied through the time constant circuit 12 to the gain-controlling ends of VCAs 9 and 10. To the input ends of expander 2, i.e., the input ends of VCAs 9 and 10, a receiver 14 is connected through a matrix circuit 13. The receiver 14 is used for receiving an output signal from the transmitter 8 through an antenna or a transmission line (dotted lines A and B).
Assume that, in the above-described construction, an L signal is supplied to an input end IN.sub.1 of the compressor 1 and an R signal is supplied to an input end IN.sub.2. L and R signals are audio signals having a specific relationship therebetween. The L and R signals are compressed by VCA 3 and VCA 4, respectively, and then supplied to the matrix circuit 7. The matrix circuit 7 generates an (L+R) signal resulting from the addition of the L and R signals, and an (L-R) signal resulting from the subtraction of the L and R signals, and the additive and substractive signals are then transmitted by means of the transmitter 8. Transmission of the additive and subtractive signals is performed so that, even if stereo broadcasting is received with a stereo receiver or a monophonic receiver, compatibility is obtained in connection with the program contents. In the monophonic receiver, only the additive signal is processed. In the stereo receiver the additive and subtractive signals received by the receiver 14 are demodulated to the L and R signals in the matrix circuit 13. The L signal is expanded by VCA 9 ad supplied to an output end OUT.sub.1, and the R signal is expanded by VCA 10 and supplied to an output end OUT.sub.2. When the compression rate of the compressor 1 and the expansion rate of the expander 2 are indicated as, .UPSILON. and .UPSILON.' respectively, and compression/expansion is performed such that .UPSILON..multidot..UPSILON.'=1, L and R signals having nearly the same characteristics as the L and R signals at the transmission side can be demodulated at the receiving side. In this way the noise level resulting from the transmission system can be controlled. The time constant circuits 6 and 12 are provided so that natural audio feeling can be obtained at the receiving side.
FIG. 2 shows an expander placed in a monophonic receiver. In FIG. 2, VCA 15 is provided between an input end IN and an output end OUT. To the input end IN is connected a level-detecting circuit 16 with an output signal therefrom being supplied through a time constant circuit 17 to the gain-controlling end of VCA 15. In a monophonic receiver, as is well known, only an additive signal is supplied to the input end IN for processing. Thus, VCA 15 expands the additive signal according to the additive signal level.
In the case, however, in which two audio signals having correlation are each compressed at the transmission side and expanded at the receiving side, as in the above-described stereo transmitting/receiving apparatus, the compression or expansion operation has been conducted according to the larger level of each audio signal. However, when such stereo broadcasting is subjected to monophonic reproduction at the receiving side, the expansion operation is performed using only the additive signal, which results from the addition of the levels of the audio signals, and therefore the fidelity of the reproduced audio signals is inevitably reduced.